1. Technical Field of the Invention
The present invention relates generally to an automotive radar system designed to emit a beam of light over a frontal scanning area to gather data used in identifying obstacles existing ahead of an automotive vehicle, and more particularly to an automotive radar system for detecting lane marks printed on a road surface and another obstacle present in front of the vehicle.
2. Background Art
In recent years, automotive radar systems are used in anti-collision systems which detect preceding vehicles and other obstacles existing on a road and white lane marks printed on the road surface and alert a vehicle operator when the vehicle is in close proximity to the obstacle or when the vehicle is going to run out of the lane mark or in automatic cruise control systems which keep the distance to the preceding vehicle constant between the right and left lane marks.
In such radar systems, detection of obstacles located at a distance of approximately 100 m requires sweeping a radar beam horizontally within a frontal zone, while detection of lane marks requires sweeping a radar beam downward to scan the road surface.
Japanese Patent First Publication No. 8-248133 discloses an automotive radar system designed to sweep radar beams both horizontally for detecting obstacles in a frontal zone, which will also be referred to as a frontal zone scanning operation below) and downward for detecting lane marks, which will also be referred to as a road surface scanning operation below). The frontal zone scanning operation and the road surface scanning operation are achieved simultaneously by splitting a single beam of light into a plurality of radar beams, directing them to a moving mirror at different vertical angles through respective reflecting mirrors, and sweeping the radar beams horizontally.
Usually, a scan beam emitted from a radar mounted on an automotive, as clearly shown in FIG. 14(a1), intersects each lane mark printed on one of sides of a road diagonally, so that an area of the lane mark to which the scan beam is irradiated is relatively small. Additionally, the smaller the incident angle .psi. of the scan beam to the road surface is, as shown in FIG. 14(a2), the greater will be an area S irradiated by the scan beam, which will cause the density of power of the scan beam on the road surface to be decreased greatly. Further, the reflectivity of lane marks is usually low as compared with reflectors mounted on typical automotive vehicles. It is, therefore, difficult for typical radar systems to detect a return of a scan beam from a distant road surface.
The above problems may be alleviated by emitting a scan beam at a great angle to the road surface so as to decrease the area S for increasing the density of power of the scan beam. This, however, requires emission of the scan beam in the vicinity of the vehicle, thereby resulting in an increase in scan angle for detecting the lane marks accurately. The increase in scan angle without reducing the resolving power of the azimuth angle of a target requires a large number of times beams are generated for each scan, thereby resulting in an increase in load of a scan beam source (e.g., laser diodes), leading to an decrease in lifetime of the scan beam source.
Further, in the above radar system designed to perform both the frontal zone scanning operation and the road surface scanning operation, the increase in angle of a scan of the road surface will also cause the angle of a scan of the frontal zone to be increased, thus resulting in undesirable detection of many safe obstacles on the sides of the road. This leads to complex signal processing for identifying target objects and an increase in operations therefor.
The sensitivity of the radar system to the lane marks may be increased by increasing the power of the scan beam source, but it will cause the lifetime of the scan beam power to be decreased greatly.